In manufacturing defined surface structures, often a plurality of method or processing steps have to be carried out one after the other. The quality of each completed intermediate product substantially affects the end product. This is why it is often necessary to examine the surface of the intermediate products. For example, in semiconductor manufacturing, wafers are sequentially processed during the manufacturing process in a great number of processing steps. As integration densities increase, the requirements as to the quality of the structures formed on the wafers become more stringent. For this purpose it is advantageous if the quality of individual processing steps, such as of lithographical steps, during the manufacturing process and prior to a downstream processing step, can be reliably assessed. Thus, if already after completion of a processing step and prior to completing the manufacturing process, it is determined that a wafer or a structure formed on a wafer is defective, the wafer can be immediately discarded, without the downstream processing steps having to be carried out. The wafer found defective can be separately post-processed until a satisfactory quality is achieved. In this way the efficiency and yield can be increased in semiconductor manufacture.
To inspect such surfaces, in particular, surfaces of wafers, optical devices are particularly suitable. Optical devices are known which are able to identify various structures on the surface of a wafer by means of image detection. Such wafer inspecting devices can operate in various illumination modes. For example, a bright-field illumination is possible by a so-called bright-field arrangement, wherein the surface of a wafer is illuminated and the light reflected from the surface is detected by a camera. A dark-field illumination is possible by a so-called dark-field arrangement, wherein the surface of the wafer is illuminated and the light diffracted by defects, particles and the like on the surface is detected by a camera. A modified dark-field illumination is implemented by the so called advanced dark-field arrangement.
Image recordings in bright-field and dark-field arrangements are of varying significance. For an optimal wafer inspection it is therefore desirable to record images with a wafer inspection device both in a bright-field arrangement and in a dark-field arrangement. To achieve high precision with the detection of defects on the surface of wafers in a dark-field arrangement, a high intensity of the light flashes used for illuminating the surface is desired. For this purpose the incident light illumination means used in the bright-field and dark-field arrangements are stroboscopically operated. They therefore emit relatively short illumination light flashes that are imaged on the surface of the wafer.
As is well known, the precision in such systems can be enhanced by carrying out color matching. To do this in a bright-field arrangement, the camera gain of the recording camera is adjusted in defined color channels, e.g. in the red and blue channels, so that the inspection device is matched with the bright-field illumination to a reference array of a wafer or a reference wafer to a predetermined color. In the dark-field mode, color matching is not possible, however. This is why erroneous detections cannot be avoided in dark-field imaging, when these are due to color deviations.